You can make changes after you have filed your tax return through the CRA or through ReFILE: https://turbotax.community.intuit.ca/replies/2647992

I don't see a Refile button in the return manager - do you need to upgrade from the free version to see this?

You should still see it in Free. Does it say that your return has been filed, or is it "In Progress"

I'm having this same problem, and I didn't prepare and file my return with the free version of TurboTax 2018 online, but the basic/standard version. There's no ReFILE button associated to my 2018 (nor my 2017) return. I retrieved the newly available rapid NOA the same day after filing my return, Is the ReFILE option actually operational yet???